1. Field of the Invention
This invention relates to image sensing apparatuses such as video cameras, or still video cameras.
2. Description of the Related Art
It has been usual to provide the image sensing apparatus such as video cameras and still cameras with an automatic exposure control device for automatically setting the exposure of the image sensor of the apparatus to an optimum value. In a case where the luminance difference between an object to be photographed and the background is large as, for example, in backlighting situations, the object image is blackened and hence without contrast details, giving an unnatural impression. To avoid this, a method of measuring light with an emphasis on a portion of the image sensing plane, for example, the interior of a frame set at the center of the image sensing plane where an object of principal photographic interest takes its place with a high probability is proposed. Another proposed method is to make use of a plurality of light measuring frames of different area size in the image sensing plane, by which an evaluative light measurement is carried out to compensate the light measurement information.
FIG. 1 is a block diagram illustrating the construction of the automatic exposure control device having a capability of compensating for backlighting by the evaluative light measurement method used in the home video camera, etc.
In FIG. 1, the video camera has a photographic lens 1, an iris 2 for regulating the intensity of entering light and an image sensor 3 such as CCD. The device further comprises a buffer amplifier 4, an AGC (Automatic Gain Control) circuit 5, a light measuring circuit 8 for performing a light measurement based on the output of the buffer amplifier 4 in accordance with a fixed-frame signal output from a fixed-light-measuring-frame signal generating circuit 10 and a compensation signal output from a backlighting compensation circuit 12, another light measuring circuit 9 which performs a light measurement based on the output of the AGC circuit 5 similarly to the light measuring circuit 8, an iris drive circuit 7 for driving the iris 2 to a size of aperture opening controlled in accordance with the output of the light measuring circuit 8, an encoder circuit 6, a video signal output terminal 13, and an input terminal 11 at which a composite synchronizing signal for setting a light measuring frame is received. Responsive to the composite synchronizing signal, the fixed-light-measuring-frame signal generating circuit 10 produces the fixed-light-measuring-frame signal for gating the video signal so that only that part of the video signal which corresponds to the position of the light measuring frame set on the image sensing plane is allowed to pass through, circuits 8 and 9 with the result that light measurement is carried out based on the signal obtained within the light measuring frame.
The device of such a construction operates as follows: The light having passed through the lens 1 and the iris 2 to the image sensor 3 is photoelectrically converted into an electrical signal. After having been subjected to signal processing such as gain control in the AGC circuit 5, the electrical signal is separated into a luminance signal and color-difference signals, which are then applied to the encoder circuit 6. The signals processed by the encoder circuit 6 are output as a composite video signal from the video signal output terminal 13. Further, in accordance with the level of the output signal of the AGC circuit 5, discrimination of whether or not backlighting is occurring is made. From the backlighting compensation circuit 12, the compensation signal is sent to the light measuring circuits 8 and 9, so that the iris 2 and the AGC circuit 5 are driven. Thus, the light measurement is compensated for the backlighting to prevent the object image from being blackened to be without contrast details by the bright background.
However, as far as the object is concerned, the backlighting compensation of the above-described automatic exposure control device ensures formation of its image without being blackened to. But, the level of that portion of the video signal which corresponds to the background of high luminance rises so abnormally as to clip or non-linearly compress the video signal. Therefore, the contract ratio of the bright portion is caused to diminish. Hence, the prior known device has a drawback of providing a picture having portions washed out.